Guillaume's "good" (3)
I believe the answer is:
bon
'guillaume's good' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bon that I've seen before include "'At the double, it's a sweet, good for the French (3)'" , "C'est good in France" , "Repeated, it's a tasty sweet" .)